AD9278BBCZ datasheet(21/45 Pages) AD

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AD9278

Data Sheet

Rev. A | Page 20 of 44

CHANNEL OVERVIEW

POST

AMP

LNA

GAIN–

GAIN+

SERIAL

LVDS

AAF

ATTENUATOR

–45dB TO 0dB

GAIN

INTERPOLATOR

PIPELINE

ADC

LOSW-x

LO-x

LI-x

LG-x

RESET

4LO+

4LO–

RFB2

GENERATION

15.6dB,

17.9dB,

21.3dB

21dB,

24dB,

27dB,

30dB

CWI+

CWI–

CWQ+

CWQ–

DOUTx+

DOUTx–

RFB1

CLG

CSH

TRANSDUCER

T/R

SWITCH

Figure 38. Simplified Block Diagram of a Single Channel

Each channel contains both a TGC signal path and a CW Doppler

signal path. Common to both signal paths, the LNA provides four

user-adjustable input impedance termination options for

matching different probe impedances. The CW Doppler path

includes an I/Q demodulator with programmable phase rotation

needed for analog beamforming. The TGC path includes a

differential X-AMP® VGA, an antialiasing filter, and an ADC.

Figure 38 shows a simplified block diagram with external

components.

TGC OPERATION

The TGC signal path is fully differential throughout to

maximize signal swing and reduce even-order distortion;

however, the LNAs are designed to be driven from a single-

ended signal source. Gain values are referenced from the single-

ended LNA input to the differential ADC input. A simple

exercise in understanding the maximum and minimum gain

requirements is shown in Figure 39.

The maximum gain required is determined by

(ADC Noise Floor/LNA Input Noise Floor) + Margin =

20 log(224/5.8) + 11 dB = 42 dB

The minimum gain required is determined by

(ADC Input FS/LNA Input FS) + Margin =

20 log(2/0.45) − 10 dB = 3 dB

Therefore, 42 dB of gain range for a 12-bit, 40 MSPS ADC with

15 MHz of bandwidth should suffice in achieving the dynamic

range required for most of today’s ultrasound systems.

The system gain is distributed as listed in Table 7.

Table 7. Channel Gain Distribution

Section

Nominal Gain (dB)

LNA

15.6/17.9/21.3

Attenuator

0 to −45

VGA Amplifier

21/24/27/30

Filter

ADC

The linear-in-dB gain (law conformance) range of the TGC path

is 45 dB. The slope of the gain control interface is 28 dB/V, and

the gain control range is −0.8 V to +0.8 V. Equation 3 is the

expression for the differential voltage, VGAIN, at the gain control

interface. Equation 4 is the expression for the VGA attenuation,

VGAATT, as a function of VGAIN.

VGAIN (V) = (GAIN+) − (GAIN−)

(3)

)

(

)

(

GAIN

ATT

VGA

−

(4)

The total channel gain can then be calculated as in Equation 5.

GAIN

ATT

GAIN

PGA

VGA

LNA

ChannelGai

)

(

(5)

In its default condition, the LNA has a gain of 21.3 dB (12×), and

the VGA postamp gain is 24 dB if the voltage on the GAIN+ pin

is 0 V and the voltage on the GAIN− pin is 0.8 V (42 dB attenu-

ation). This results in a total gain (or ICPT) of 3.6 dB through

the TGC path if the LNA input is unmatched or a total gain of

−2.4 dB if the LNA is matched to 50 Ω (RFB = 350 Ω). However,

if the voltage on the GAIN+ pin is 1.6 V and the voltage on the

GAIN− pin is 0.8 V (0 dB attenuation), the VGA gain is 24 dB.

This results in a total gain of 45 dB through the TGC path if the

LNA input is unmatched or in a total gain of 39 dB if the LNA

input is matched.

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